The present invention is directed to a femur endoprosthesis for an artificial hip joint, which is an alternative to the femur endoprosthesis in accordance with German patent 196 01 340 (U.S. patent application Ser. No. 08/783,476, the disclosure of which is incorporated herein by reference).
Essentially two treatment possibilities have proven themselves during recent decades for treatment of a destroyed hip joint head from which considerable pain sensations and serious functional restrictions of the joint result:
First, it is possible to provide the patient with an orthotic device, that is with an external support apparatus. In addition to the fact that this therapeutic possibility is completely unacceptable from a cosmetic point of view, it is also less than optimal from the perspective of therapeutic technology.
A further, more widely accepted possibility is to provide the patient with an endoprosthesis, namely a hip stem endoprosthesis, in which a stem is installed in the previously excavated bone marrow area of the femur and is there fixed with or without the use of cement. Such an endoprosthesis then offers the possibility of connection on its proximal side with an artificial spherical joint part.
It is precisely the latter possibility which is strongly represented in patent literature. At this point, reference is made only to DE-U-94 12 408 U1 as an example.
There are some approaches which are quite promising, with respect to long-term stability in the patient's body, among the various hip stem endoprostheses proposed in the patent literature. Sooner or later, however, for example after 10 to 15 years, subjection of the endoprosthesis to wear and tear is to be observed, which leads to the necessity of a revision intervention to remove the implanted endoprosthesis and replace it by a new one. This is certainly not wholly without problems, since a considerable portion of the natural bone material is removed in the initial implantation by the resection and clearing of the bone marrow area. The material once removed is then under the circumstances absent in a revision intervention. This then especially has the effect that, when the patients are relatively young, which one must assume from the beginning, they will be subjected to at least one revision intervention in the course of their lives.
An endoprosthesis which manages with a smaller resection of natural bone material than the known hip stem endoprostheses has become known from DE-A-27 24 234 C2. The prosthetic element described therein is implanted in the upper region of a femur, but below the lesser trochanter, without a stem extending into the bone marrow area. This prosthesis, which is also designated as a pressure plate prosthesis, basically reaches with a proximal pressure plate over the cortex of the resected femur in the area of the removed hip joint head. It is braced with a pressure plate, which lies laterally on the femur, in such a manner that all mechanical forces between the endoprosthesis and the femur are directly introduced into the cortical layer of the femur, whereby a mechanical strain on the spongiosa, which is felt to be impermissible, should be avoided. Only a small amount of bone cement is necessary for fixing the pressure plate in the proximal region.
As explained, underlying the philosophy of this prosthesis is the assumption that the spongiosa of the femur should be subjected to as little strain as possible. This is achieved at the expense of an extreme load upon the cortex of the femur, since namely all forces are loaded onto it and introduced into it. This in no way corresponds to natural conditions according to current knowledge.
Moreover, WO 89/11837 shows a prosthesis which operates quite similarly, in which the function of the proximal pressure according to the previously mentioned publication is undertaken by the specially constructed hip joint head, which is provided with an interior recess on which the resected femur stump is installed in the interior with creation of pressure.
Two prostheses, in which the main burden of the spongiosa of the femur bone is to be assumed, have become known from FR 26 26 169 A1 and FR 26 74 122 A1. In the first of the two named publications, there is provided a proximal threaded bolt, having a plug-in cone, for screwing into the spongiosa. This may lead to severe instability problems within a very short time. In the second of the two named publications a plate bearing a plug cone is fastened by means of a series of bone screws, wherein the bone screws reach into the spongiosa. Serious stability questions also arise from this, since the spongiosa by its nature can only be subjected to slight point load in comparison with the cortex of the femur bone.